This invention relates to sealing of feedthrough assemblies for ceramic or metal vessels, and in particular to niobium-ceramic feedthrough assemblies for such vessels for high temperature and high pressure or high temperature and vacuum applications, and processes for sealing same which preserve the ductile properties of the niobium.
An example of an application of the invention is the use of the feedthrough assembly according to the invention as an electrical feedthrough and sealable fill opening in ceramic lamp envelopes, for example high pressure discharge arc tubes for HID (high intensity discharge) lamps such as high pressure sodium lamps. Alumina is a preferred lamp envelope material for such lamps due to its translucence, thermal shock resistance, and corrosion resistance. Conveniently, alumina is also used to form sealing inserts hermetically sealed to and closing each end of an alumina lamp envelope tube. Alternatively, the alumina insert may seal a sapphire lamp envelope. Yttria is another possible lamp envelope and insert material, having properties similar to those of alumina.
In general, an HID lamp is constructed with a niobium feedthrough tube extending through an axial opening in each sealing insert, the niobium feedthrough being hermetically sealed to the alumina of the insert. Niobium is selected as the material for the feedthroughs because the coefficient of thermal expansion of niobium matches that of alumina over a wide range of temperatures and because the materials are chemically compatible. Tungsten electrodes extend into each end of the arc tube through the niobium feedthroughs. The electrodes are TIG (tungsten inert gas) welded to the niobium to make a hermetically sealed electrode-feedthrough assembly. The lamp is dosed with the desired lamp fill materials prior to the closing off of the second niobium feedthrough carrying the second electrode.
One type of prior art niobium-alumina seal involves the use of a ceramic sealing frit. A feedthrough assembly is first formed by welding the tungsten electrode to the niobium tube. The feedthrough assembly is then bonded to the alumina end seal using a ceramic sealing frit. One example of such a ceramic sealing frit is disclosed in U.S. Pat. No. 3,441,421, in which a composition of calcia, magnesia, and alumina is used to form a seal at a temperature of about 1400.degree.-1500.degree. C. However, under normal frit processing conditions, the niobium feedthroughs exhibit grain growth and recrystallization, which affect somewhat their ductility. This decrease in ductility can cause premature cracking, limiting lamp life.
Another type of seal is a brazed seal utilizing metals or eutectic metal alloys to form the braze. Such seals are described in, for example, West German Patent No. 1,013,216, in which a thin layer of metal such as Ag, Au, Cu, Ni, Fe, Co, or Mn, or alloys thereof is added to a layer of an active metal such as Ti or Zr, dissolving a portion of the active metal during the brazing process.
The processing temperatures, atmosphere, and composition of brazed seals can also result in unacceptable long term embrittlement of the niobium feedthroughs. This and the above-described ceramic frit sealing method also limit the cold spot or end temperature to 800.degree. C. due to the softening temperature of the sealing alloys, and can introduce new phases during processing which may be reactive with certain lamp fills, for example metal halides and active metals.
Direct niobium-to-ceramic seals are disclosed in U.S. Pat. No. 4,545,799, incorporated herein by reference. The assembled inserts and lamp envelope are partially sintered, the niobium feedthroughs are inserted into the axial openings in the inserts, and the assembly is fully sintered to translucency, forming the seal as the insert material shrinks during the sintering process. This process is superior to prior processes since it permits a higher cold spot temperature than the fritted or brazed seals. A Hg lamp with a ceramic arc tube has been operated with end temperatures at 1200.degree. C. The loss of ductility in the niobium resulting from this process may be tolerated when the niobium feedthroughs are closed off by capping. U.S. Pat. No. 4,545,799 describes a direct seal having a niobium cap welded to the feedthrough, the electrode being welded to the inside surface of the cap. It would be of great advantage, however, to further simplify the sealing process by eliminating this additional step of welding a cap to the feedthrough.
A lamp assembly having a pinched-off feedthrough, or a feedthrough which is first pinched off then welded, would greatly simplify the lamp assembly process. However, pinching-off of the feedthrough requires greater ductility in the material at the outer end of the feedthrough than does the feedthrough capping process. The present invention provides such a ductile pinched-off assembly, as well as a sealing process for achieving the required ductility at the outer end of the feedthrough.
The feedthrough assembly and sealing process according to the invention is useful for forming fritless, frit, brazed, or other seals where niobium feedthroughs are exposed to high temperatures. This improvement is due to the improved mechanical properties of the niobium. The invention is not, however, limited to use in the lamp industry, but is useful whenever a hermetic seal is desired around a niobium feedthrough, rod, wire, or other piece extending through a ceramic sealing insert or other ceramic sealing means, for example in an electrical, high pressure, or vacuum feedthrough or port assembly through the wall of a metal or ceramic vessel.